The present invention relates to medical devices, and, more particularly, to implantable medical device.
Impedance monitoring has been used with implanted devices and in external monitoring devices for determination of numerous physiologic conditions. Transthoracic impedance measurements give a good indication of the fluid status of patients, with decreases in impedance being indicative of increases in fluid content. Knowledge of a patient's long-term impedance measurements and changes therein are a valuable clinical indicator of a patient's health.
While a possible indication of other conditions, the accumulation of fluid can also indicate failing heart circulation. There are several mechanisms or diseases that can cause or affect the accumulation of fluid. In general, fluid accumulation is a failure or over-response of the homeostatic process within the body. The body normally prevents the build up of fluids by maintaining adequate pressures and concentrations of salt and proteins and by actively removing excess fluid. Fluid accumulation can occur, for example, when the body's mechanisms for preventing fluid accumulation are affected by disease, such as heart failure, left sided myocardial infarction, high blood pressure, altitude sickness, emphysema (all which affect pressures), cancers that affect the lymphatic system, and diseases that disrupt the protein concentrations. As a result, providing an adequate monitor of the patient's fluid status can provide physicians and patients with a better tool to manage disease.
However, impedance monitoring sensitivity to intestine and other peripheral edema is not currently known. It is speculated that impedance monitoring during the time when a patient is upright, for example noon till 5 p.m., normalizes postural changes that would otherwise confound the impedance measurement. Further, upright monitoring may also mask earlier warning signs that may be evident if the impedance monitoring was performed in other patient body positions.
Presently there are devices which track impedance changes in a patient over time and monitor for deviations that would be consistent with fluid overload. To maintain good specificity the algorithm only uses data collected between 12 pm and 5 pm each day. This standardization is intended to eliminate postural variability that could confound the trending algorithm.
Patients who are retaining too much fluid have a greater problem with pulmonary congestion when supine as opposed to when standing or sitting. Many patients for example must sleep with multiple pillows elevating their head to displace fluid to their lower extremities. As more fluid is retained this problem becomes greater, as the extremities are less capable of absorbing the volume.